A suspected Russian oil tanker has been detained in the Atlantic, France has announced, in the latest seizure aimed at combatting Moscow’s “shadow fleet” of vessels contravening international sanctions.

The Tagor was detained on Sunday morning in international waters more than 400 nautical miles (740km) west of Brittany with the help of the UK and other partners, said the French president, Emmanuel Macron.

According to French authorities, the vessel was on its way from Murmansk in north-west Russia when it was seized.

Macron said the French navy boarded an oil tanker that was subject to international sanctions and sailing from Russia. He posted a video he said was of the seizure, which showed commandos descending from a helicopter on to a ship.

Macron wrote on X: “This operation took place in the Atlantic Ocean, on the high seas, with the support of several partners, including the United Kingdom, in strict compliance with the law of the sea.”

He said: “It is unacceptable for ships to circumvent international sanctions, violate the law of the sea, and finance the war that Russia has been waging against Ukraine for more than four years.”

The Kremlin spokesperson, Dmitry Peskov, said of the ship’s seizure: “We consider these acts as illegal, they border on international piracy … Russia is taking measures to ensure the safety of its cargo.”

The ship was falsely flying a Cameroonian flag and was heading towards Limbe, a coastal city in the west of the African country, a spokesperson for the French maritime prefecture told Agence France-Presse.

The Atlantic maritime prefecture said: “This operation was aimed at checking the nationality of a vessel suspected of flying a false flag. After the inspection team boarded the vessel, an examination of the documents confirmed suspicions regarding the irregularity of the flag flown. In accordance with international law and at the request of the public prosecutor, the vessel was diverted.”

The ship, which had 23 crew members, was “being escorted by the French navy to an anchorage point for further checks”, it added.

Guillaume Le Rasle, a spokesperson for the prefecture, said the tanker was under EU and US sanctions. “It is a vessel that was known and tracked,” he told AFP.

“The decision to divert it was taken Sunday evening. The objective of the diversion is to verify the validity of its flag,” Le Rasle said, adding that the tanker, which has frequently changed flags, was “almost empty” at the time of boarding.

Russia’s embassy in Paris said on Monday it had requested information from the French authorities about any Russian citizens onboard the Tagor, the state news agency Tass reported.

The captain of the vessel is, according to preliminary information, a Russian citizen, the embassy said.

France and the UK have vowed to obstruct ships linked to Russia’s sanctions-hit “shadow fleet” that pass through their waters. The British prime minister, Keir Starmer, announced in March that he had granted permission for the UK military to board ships belonging to the shadow fleet.

However, shipping data shows that dozens of ships linked to Russia and subject to sanctions continue to cross UK waters.

The shadow fleet vessels frequently change the flags they fly, a practice known as flag-hopping, or use invalid registrations in an attempt to escape tracking. Since September, France has boarded three other ships believed to belong to Russia’s shadow fleet. The vessels were allowed to sail after their owners paid fines.

In September, the French navy boarded the Boracay, which claimed to be flagged in Benin. Its Chinese captain was put on trial in absentia, and a French court in March issued an arrest warrant and a one-year jail sentence against him.

In January, French forces impounded another suspected Russian tanker, the Grinch, and in March, the Deyna, which had sailed from Murmansk under a Mozambican flag, was detained in Marseille.

In April, France announced a plan to double penalties for ships that fail to fly a flag or refuse to comply.

Several western countries have imposed sanctions on hundreds of vessels in Russia’s shadow fleet over its 2022 invasion of Ukraine. Nearly 600 vessels suspected of being part of that fleet are subject to EU sanctions.

Agence France-Presse and Reuters contributed to this report